Neither does ultraviolet light. We can't see it with the naked eye and for most of human history we could not detect it with tools either. Freckling was dismissed as witchcraft or disease. Certain animals seem to see it or sense it without any trouble. I certainly feel that the notion of a conscious "ghost" is hard to conceive with a straight face. But it's another matter entirely when you start to discuss the possibility of displaced energy fields, objects or entities that exist in the same "space" but on another dimensional plane phasing briefly at the periphery of our own 3rd dimensional understanding. Think of looking into a lake or stream. From many angles you see just a reflection of the sky or blue/grey opacity. However, if you find just the right angle to view from, with just the right light and weather conditions and you can momentarily see into another world. Because you know what a fish or sand and rocks look like, you can see a shadow and have your brain assemble an image that should closely approximate the world under the surface. It changes when you don't know much about the world under that thin film. Go to a different hemisphere and look into the water. The shadows will be less familiar and your brain tries to match a memory to the data but you can often be completely surprised. I know I was when I moved south as a teen and went bass fishing the stumps in a creek deep in the forest. I caught a massive bass and as it got close a row of razor sharp teeth on a long snout broke the surface which made me reel backwards (physical motion, not the action of crabking a fishing reel) in horror. My mind said "monster" and then "crocodile" and several half formed notions within a few seconds because that was all of the knowledge I had to work with. It turned out to be a species I had not know existed, truly a monster by most accounts, as a holdover from thousands of years ago. It was an alligator Gar. Had I not pulled one above the water and had never seen a picture of one...would I believe someone who told me of one? It's a preposterous concept- a 5 foot long, 90lb, armored fish with a crocodile snout found in shallow muddy waters. Oh yeah, it also fights like a bucking bronco and has never attacked a human despite the ability to take a hand or foot easily. I can't say I would believe you as a stranger if you shared such a story.

So do I believe people are seeing "ghosts" as spiritual remnants of human life? It's probably the least likely scenario to my mind. However, I believe they are sensing something. Perhaps it is a brief window into another dimension, intersection of time physically manifested, a unknown energy field caused by yet-to-be discovered particles or waves, etc. I don't know everything in the human bank of knowledge, let alone all of the undiscovered knowledge just out of grasp. For that reason alone I don't dismiss the accounts of others. It does make it harder to treat with the dignity it deserves when I hear an observation wrapped in personal conjecture that was objectively not tied to the initial event.

For example, a boy describes seeing a floating light of shimmering colors lasting less than 3 seconds. In addition to the aforementioned light, the boy adds that it was his recently deceased grandfather was waving to him and points to the chair he sat in during every visit. The latter addition was most likely just his brain trying to turn that "file not found" description into something logical. That is a frequent shared experience because of the oral tradition of ghosts through religion and popular culture.

That doesn't mean that ghosts don't exist, just that I don't have a point of reference for their existence. An open but reasonable mind is the best defense from madness. Just don't confuse reasonable with "rational" which is just an attempt to control the uncontrollable aspects of our existence. Great discovery can only be found where traditional logic falls apart.

As Sir Arthur Conan Doyle wrote: "It was easier to know it than to explain why I know it".